[codebje]

CQRS should really only guide you to designing separate query and command interfaces. If your processing is asynchronous then you have no choice but to have state about processing-in-flight, and your commands should return an acknowledgement of successful receipt of valid commands with a unique identifier for querying progress or results. If your processing is synchronous make your life easier by just returning the result. Purity of CQRS void-only commands is presentation fodder, not practicality.

(One might argue that all RPC is asynchronous; all such arguments eventually lead to message buses, at-least-once delivery, and the reply-queue pattern, but maybe that's also just presentation fodder.)